The present invention relates generally to a process of manufacturing an electrochemical device, and more particularly to an improved process of manufacturing an electrochemical device of laminar structure comprising solid electrolyte layers.
There have been known various electrochemical devices (electrochemical sensing elements) using solid electrolyte, for example, oxygen sensors to detect the oxygen concentration of an exhaust gas emitted from internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include an oxygen sensor which comprises a body of oxygen-ion conductive solid electrolyte such as zirconia ceramics and which operates to determine the oxygen concentration according to the principle of an oxygen concentration cell. Also known in the prior art are electrochemical devices such as sensing and pumping elements or cells for hydrogen, nitrogen, carbon dioxide, etc. In such electrochemical cells, solid electrolyte materials have been generally used in the form of a tubular body which has an elongate bore closed at its one end. In recent years, however, it has been attempted to replace the tubular solid electrolyte body with a solid electrolyte body of planar shape as disclosed in U.S. Pat. Nos. 4,334,974; 4,282,080; and 4,300,990, in view of relatively low productivity and high cost of manufacture of solid electrolyte bodies of tubular type, and for the benefit of easy assembling of parts with a planar solid electrolyte body to form an electrochemical device. When such planar solid electrolyte bodies (i.e., solid electrolyte in the form of layers or sheets) are employed, suitable electrodes are disposed on the surfaces of the layer or layers of solid electrolyte, and the electrolyte layers and other layers or parts are assembled as a stack into a laminar structure constituting an electrochemical device or sensing element.
One exemplary form of a known electrochemical device of laminar structure comprises an electrochemical pumping cell having a first and a second electrode disposed on a planar solid electrolyte body, and an electrochemical sensing cell having a third and a fourth electrode disposed on another planar solid electrolyte body. These pumping and sensing cells are constructed such that the second and fourth electrodes (inner pumping and measuring electrodes) are exposed to the atmosphere in an internal cavity that is defined by the above solid electrolyte bodies and usually also by a spacer member which is sandwiched by the solid electrolyte bodies and which may also be a solid electrolyte body. For easy manufacture of such an electrochemcial device, it is proposed to employ a process which comprises the steps of: superposing unfired planar solid electrolyte bodies (green sheets or unfired layers of solid electrolyte) in a stack such that a cavity of a suitable volume is formed within the stack; forming unfired layers of electrodes on the green sheets of solid electrolyte as required; applying a pressure to compress the laminated stack of the solid electrolyte and electrodes to form an unfired integral laminar structure; and co-firing the laminar structure at a suitable sintering temperature of the solid electrolyte.
In the above introduced manufacturing process wherein the unfired green sheets of solid electrolyte are laminated under pressure into an integral laminar structure, the internal cavity formed by the green sheets during the laminating step is more or less deformed due to the compressing pressure applied to the green sheets. As a result, the cavity formed in the fired laminar structure of one electrochemical device tends to have different volume and shape from those of the cavity formed in the other devices. Different volumes and shapes of the cavities lead to inconsistent operating characteristics and performance between the electrochemical devices, thereby causing these devices to fail to satisfy the practical requirements. Although the aforementioned deformation (reduction in volume) of the internal cavity may be avoided by reducing the pressure to be applied to the stacked assembly, the reduced pressure causes difficulty in attaining sufficient mutual adhesion or bonding of the individual green sheets of solid electrolyte, and consequently leads to a decline in the yield of the products. Hence, the adoption of such solution to the above problem is far beyond possibility.